The Christmas star is here! At least that's how it will seem in early December, as a brilliant light hangs low in the south west skies at dusk: 20 times brighter than any star, and second only to the Moon in lighting up the night sky.

Despite its seasonal appearance, this is not, in fact, the Star of Bethlehem. Astronomers still argue over the nature of the star that presaged Christ's birth: front-runners are a close approach of Jupiter and Saturn, or perhaps a stellar explosion (a nova). What we are seeing now is more mundane occurrence: an appearance of our nearest neighbour planet, Venus.

Through a small telescope - or even good binoculars, held steadily - you'll see Venus shaped as a crescent, as the Sun lights up its globe obliquely. But even the most powerful telescope reveals little more. Venus hides her secrets under an all-enveloping cloak of brilliant clouds.

Space probes have unveiled the mystery planet by using radar waves to penetrate the clouds. The most detailed radar views have come from Nasa's Magellan mission, which orbited Venus as an artificial moon. The latest result is the discovery of narrow, winding river-beds. The longest of the channels is Baltis Vallis, which bears the Syrian name for Venus. It winds for 6,800km, longer than any river on the Earth. And its sediments have covered a plain twice the size of the US.

But Baltis Vallis cannot be the bed of a river such as we know on Earth. For billions of years, the planet has been sweltering under a thick atmosphere of carbon dioxide, creating a greenhouse effect that has raised its temperature to 460 C. Water cannot flow on Venus: it would boil away instantly.

Venus is also a highly volcanic world, so one obvious culprit is a flow of lava. But ordinary lava is too viscous to carve such a thin and sinuous channel. And the planet's temperature - though hot for us - is comparatively cool for lava; it would solidify quite quickly, at least on the surface, and disappear into a subterranean lava tube.

Applying the Sherlock Holmes maxim that "When you have eliminated the impossible, whatever remains, however improbable, must be the truth", the Nasa team is now suggesting that the river consists of an incandescent, flowing stream of melted washing soda and chalk.

This is not quite as implausible as it sounds. The Earth has one volcano that erupts molten carbonates: Oldonyo Lengai in Tanzania. This type of lava - known as carbonatite - is much more runny than ordinary lavas, which are made of silicate rocks (such as basalt). And carbonatite can cool to almost the temperature of Venus's surface before solidifying, so there's no problem explaining how it can flow so far.

But why should Venus's volcanoes have such large supplies of carbonate rocks? Perhaps in the planet's early days it had a milder climate and vast oceans. The carbonates were laid down on the seabed, like chalk and limestone on Earth. Then the greenhouse effect took over, and the oceans boiled away. Venus's volcanoes have now melted its store of carbonates, and in the new hothouse climate they have flowed fast and far, to form the longest "rivers" in the Solar System.

More information about Venus can be found on the World Wide Web at http://pds.jpl.nasa. gov/planets/welcome/venus.htm

What's up

To the upper left of Venus lies the second-brightest planet - the gas giant Jupiter.

Over to the south, look out for the rather dimmer, ringed world of Saturn. In the east, the bright constellations of winter are rising: Orion (the hunter), Taurus (the bull) and Gemini (the twins), along with Canis Major and Minor (the two dogs).

The night of 13 December sees the annual Geminid shower of shooting stars, but this year the display will be marred by bright moonlight.